1. Field of the Invention
This invention relates to a weld control for spot welding workpieces of various metals that relies on the time rate of change of resistance between the welding electrodes. More particularly, the invention pertains to a method for determining shortly after circuit variables have stabilized at the beginning of the welding process the optimum weld current or weld schedule on the basis of the time rate of resistance change.
2. Description of the Prior Art
It is known that in resistance welding of metals there is a discernable change in the electrical resistance of the workpiece as the weld is made. During the first few cycles in which electrical energy is applied to the welding electrodes, the electrical resistance across the electrodes is unstable because it is affected by the conditions of the material surfaces. After this period of instability is passed, the resistance under normal welding conditions gradually rises as the temperature of the workpiece rises until the onset of fusion. Most importantly the resistance reaches a peak value and then falls due to fusion and the indentation of the metal caused by the force applied to the welding electrode.
A spot weld is generated by a dynamic process wherein the metal is melted initially on the axis equidistant from the electrodes and then axially toward the electrodes and radially outward from the axis. The electrodes apply a clamping force to the workpiece which confines the melt. The weld process is stopped by terminating the welding current before the melt exceeds the electrode diameter. Otherwise, an impressive but totally undesirable shower of sparks and hot metal will issue from the weld spot. The magnitude of the weld current and the resistivity of the material of the workpiece determine the speed at which a weld nugget is produced. Metal expulsion that produces the shower of sparks sets one upper limit for the current; however, when weld current is too low, merely lengthening the period during which power is supplied to the electrodes may not produce an acceptable weld nugget. The electrical resistivity of the workpiece is an important factor affecting nugget size for a given weld current. The temperature at which each material enters the plastic range where workpiece indentation begins is another important variable.
The principal factors affecting the quality of a spot weld include the magnitude of the clamping force applied to the electrodes that hold the workpiece in position, the duration of the clamp time, the duration of the period during which electrical energy is applied to the electrodes, the magnitude of the weld current and the duration of the holding time during which force is applied to the electrodes after the current is terminated and the weld is made. These factors are adjusted during testing so that an appropriate weld schedule for a particular thickness and kind of weld material is found to produce an optimum weld in the shortest period of time. There are however other variables whose importance is difficult to access in defining the optimum weld schedule but which certainly have a considerable effect on the quality of the weld produced. For instance, the current shunt path through one or more completed adjacent welds operates to draw current from the location where the weld is being made. In this instance, the indicated weld current is greater than that which is actually applied to the weld spot. Another variable is the increasing diameter of the soft copper electrode that results from the heat and pressure of welding and repeated use. Other variables are line voltage variations, multiple thickness workpieces and the use of materials having different current level requirements. The material properties of the workpiece, particularly its chemical composition, have a pronounced effect on the quality of the weld and cannot be correctly evaluated by adjusting the few variables that are usually measured and used by conventional spot weld controllers.
Adaptive controls have been employed to control the weld process of randomly variable workpiece materials by sensing other variables in the process. For example, the temperature inside the weld nugget is an important indication of weld quality but it cannot be measured directly. The emission of sound produced during welding is susceptible to noise interference but it is not coupled to or a part of the weld process mechanism. Thermal expansion of the workpiece has been used for control but it can not be practically applied in portable welders.
Time-adaptive welding controls that sense the resistance between the electrodes have been used to maintain the application of welding current for an extended period until the resistance of the weld decreases by a predetermined percentage, usually about five to ten percent below its peak value, after which the electrical power is disconnected from the electrodes. Time-adaptive controls of this type are limited when the current is too low to produce a significant rise and drop in resistance. When this condition exists the weld must be stopped at a certain predetermined maximum time but only a very small weld nugget, or none at all, will be produced. Time-adaptive controls work best when ample welding current is applied. Such controls compensate for current shunting, changes in electrode size, some variation in workpiece thickness and minor voltage fluctuations.
Time-adaptive controls are not by themselves sufficient to permit the welding of widely different steel alloys, such as mild steel and phosphorized steel, with the same control setting for the weld current. The operator would have to know the type of material beforehand and change the settings of weld current manually to the appropriate value for the particular workpiece material.